LM4883SQ datasheet(17/23 Pages) NSC

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Application Information (Continued)

placing the LM4883 in bridged mode operation. The output

coupling capacitor blocks the amplifier’s half supply DC volt-

age, protecting the headphones.

The HP-IN threshold is set at 4V. While the LM4883 operates

in bridged mode, the DC potential across the load is essen-

tially 0V. Therefore, even in an ideal situation, the output

swing cannot cause a false single-ended trigger. Connecting

headphones to the headphone jack disconnects the head-

phone jack contact pin from −OUTA and allows R1 to pull the

HP Sense pin up to V

DD. This enables the headphone func-

tion, turns off Amp A (+out) and Amp B (+out) which mutes

the bridged speaker. The amplifier then drives the head-

phones, whose impedance is in parallel with resistors R11

and R12. These resistors have negligible effect on the

LM4883’s output drive capability since the typical impedance

of headphones is 32

Ω.

Figure 4 also shows the suggested headphone jack electri-

cal connections. The jack is designed to mate with a three-

wire plug. The plug’s tip and ring should each carry one of

the two stereo output signals, whereas the sleeve should

carry the ground return. A headphone jack with one control

pin contact is sufficient to drive the HP-IN pin when connect-

ing headphones.

A microprocessor or a switch can replace the headphone

jack contact pin. When a microprocessor or switch applies a

voltage greater than 4V to the HP-IN pin, a bridge-connected

speaker is muted and Amp A (-out) and Amp B (-out) drive a

pair of headphones.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4883’s performance requires properly se-

lecting external components. Though the LM4883 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4883 is unity-gain stable, giving a designer maximum

design flexibility. The gain should be set to no more than a

given application requires. This allows the amplifier to

achieve minimum THD+N and maximum signal-to-noise ra-

tio. These parameters are compromised as the closed-loop

gain increases. However, low gain demands input signals

with greater voltage swings to achieve maximum output

power. Fortunately, many signal sources such as audio

CODECs have outputs of 1V

RMS (2.83VP-P). Please refer to

the Audio Power Amplifier Design section for more infor-

mation on selecting the proper gain.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitors (C1–4) in Figures 1, 3. A high value

capacitor can be expensive and may compromise space

efficiency in portable designs. In many cases, however, the

speakers used in portable systems, whether internal or ex-

ternal, have little ability to reproduce signals below 150 Hz.

Applications using speakers with this limited frequency re-

sponse reap little improvement by using large input capaci-

tor.

Besides effecting system cost and size, C1–4 have an effect

on the LM4883’s click and pop performance. When the

supply voltage is first applied, a transient (pop) is created as

the charge on the input capacitor changes from zero to a

quiescent state. The magnitude of the pop is directly propor-

tional to the input capacitor’s size. Higher value capacitors

need more time to reach a quiescent DC voltage (usually

DD/2) when charged with a fixed current. The amplifier’s

output charges the input capacitor through the feedback

resistors, R2,3,7,and 8. Thus, pops can be minimized by

selecting an input capacitor value that is no higher than

necessary to meet the desired −3dB frequency.

A shown in Figure 3, the input resistors (R1,4,5, and 6) and

the input capacitors, C1–4 produce a −3dB high pass filter

cutoff frequency that is found using Equation (7).

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, C

1, using Equation (7) is 0.053µF. The .33µF

1 shown in Figure 3 allows the LM4883 to drive high

efficiency, full range speaker whose response extends below

30Hz.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of C

5, the capacitor con-

nected to the BYPASS pin. Since C

5 determines how fast the

LM4883 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4883’s

outputs ramp to their quiescent DC voltage (nominally 1/2

DD), the smaller the turn-on pop. Choosing C5 equal to

1.0 µF along with a small value of C

1 (in the range of 0.1 µF

to 0.39 µF), produces a click-less and pop-less shutdown

function. As discussed above, choosing C

1 no larger than

necessary for the desired bandwith helps minimize clicks

and pops.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4883 contains circuitry that minimizes turn-on and

shutdown transients or “clicks and pop”. For this discussion,

turn-on refers to either applying the power supply voltage or

when the shutdown mode is deactivated. While the power

supply is ramping to its final value, the LM4883’s internal

amplifiers are configured as unity gain buffers. An internal

20088724

FIGURE 4. Headphone Circuit

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